Method for the blackening of aluminum alloys

ABSTRACT

An even, tenacious black coating is applied to the surface of copper-containing aluminum alloys by immersion of the alloy into an aqueous bath containing from 10-20 g/l of an alkali, metal bichromate and from 8-50 g/l of an alkali metal hydroxide for a period up to 1.5 minutes according to the disclosed process. Preferably the aqueous bath also contains from 30-40 g/l of an alkali metal carbonate.

United States Patent [191 Boose et al.

METHOD FOR THE BLACKENING OF ALUMINUM ALLOYS Inventors: Cesar Adrianus Boose, Hague; Maarten Johan Reidt, Voorburg; Evert Hermanus Lodewijk Derksen,

Appl. No.: 123,051

US. Cl. ..148/6.2, 148/627, 148/315 Int. Cl. .Q ..C23f 7/26 Field of Search .....1 48/6.2, 6.27, 31.5

[1 11 3,725,137 51 Apr. 3, 1973 [56] References Cited UNITED STATES PATENTS 1,838,633 12/1931 Pacz ..l48/6.27 X 2,493,934 1/1950 Waring ....148/6.27 X 2,544,139 3/1951 Primary Examiner-Ralph S. Kendall Attorney-Cushman, Darby & Cushman [5 7] ABSTRACT An even, tenacious black coating is applied to the surface of copper-containing aluminum alloys by immersion of the alloy into an aqueous bath containing from 10-20 g/l of an alkali, metal bichromate and from 8-50 g/l of an alkali metal hydroxide for a period up to 1.5 minutes according to the disclosed process. Preferably the aqueous bath also contains from 30-40 g/l of an alkali metal carbonate.

6 Claims, No Drawings Deyrup et al ..148/6.2

METHOD FOR THE BLACKENING OF ALUMINUM ALLOYS The invention relates to currentless blackening of a copper-containing aluminum alloy by immersion in a bath containing alkali bichromate.

It has been found that the above-mentioned method was not a good and cheap process for blackening the aluminum alloy 7075, also known as perunal. The aluminum alloy 7075 may be stated as follows:

Cu 1.3 1.8 Si (Fe Si) max. 0.4 Mn 0.1 0.4 Mg 2.1 2.5 Zn 6.0 65 Cr 0.1 0.3 Ti 0.03- 0.l5

The specific gravity of perunal is 2.80.

By good and cheap blackening of perunal is meant a black surface coating which conforms to the following four requirements:

1. the emission coefficient of the coating must be greater than or equal to 0.8 for temperature radiation between 20 and 100 C;

2. as a criterion for adhesion the coating must be able to pass a tape test;

3. cheap in application;

4. very uniform.

The invention aims at indicating a good as well as cheap method of applying a .black surface coating to copper-containing aluminum alloys, more particularly alloys of the perunal type, which coating conforms to the aforementioned four requirements.

1 According to the invention a bath is used which contains quantities of alkali carbonate, alkali bichromate and alkali hydroxide corresponding to the undermentioned composition:

potassium bichromate sodium hydroxide In order to obtain good results with the above-mentioned bath the immersion times should be up to 1% minutes, and the bath should have a temperature between 80 to 100C.

The alkali metal bichromatesand hydroxide applicable' are in principle all alkali metals, 7 provided equivalent weights are allowed, corresponding to the above-mentioned composition of the bath.

In view of cost-price considerations, however, rubidium and cesium compounds are not used in practice.

In some cases the result of the blackening bath can be improved by the addition of a quantity of alkali carbonate to the immersion bath. The quantity of alkali carbonate corresponds to a sodium carbonate concentration of between 30 and 40 g per liter.

Very good blackening results are obtained with bathing times up to 1 minute and bath temperatures between 95 and 100 C and the undermentioned bath composition:

sodium carbonate 35 g/l potassium bichromate l5 gll sodium hydroxide 10 g/l The invention is elucidated in the three examples now following.

Example I deals with the operation of covering perunal with a grey surface coating in a bath containing sodium carbonate and potassium bichromate.

Example II deals with the treatment of perunal in a bath also containing alkali hydroxide in addition to alkali bichromate and alkali carbonate.

Example III deals with the blackening of perunal, using a bath containing only alkali bichromate and alkali hydroxide.

For Examples I, II and Ill, pieces of perunal 4 mm thick and with dimensions of 5 X 2 cm were successively freed of grease, treated with an alkaline pickling agent, rinsed and given a clean surface with the aid of a smut remover.

The removal of grease was effected in a greaseremoving bath of the following composition and bath conditions:

sodium gluconate I50 g/l trisodium phosphate 50 g/l temperature C bathing time 5 minutes.

Alkaline pickling was effected under the following conditions:

sodium hydroxide 50 g/l sodium gluconate lOO g/l temperature 60 70 C bathing time 1 2 minutes.

EXAMPLE I.

Pretreated clean-scoured perunal plates were im-' mersed for 5 10 minutes in a bath with the undermentioned compositionand under the following conditions:

sodium carbonate 35 g/l potassium bichromate 15 g/l temperature C pH about 10.

With bathing times of from 5 to 10 minutes, and slight gas development, grey surfaces showing excellent adhesion were obtained. In regard to this it is observed that the surface coating obtained did "not show the requisite blackness.

EXAMPLE II.

To the immersion bath according to Example I a quantity of 10 g/l of sodium'hydroxide was added.

With bathing times up to 1 minute, under the same conditions as in Example 1, black surface coatings showing good adhesion were obtained.

Longer bathing times had an adverse effect upon adhesion, as did also higher sodium hydroxide concentrations.

EXAMPLE III.

It was found that pretreated clean-scoured perunal plates could be well blackened by being immersed in a bath with the following composition and bath conditions:

sodium hydroxide 47 g/l potassium bichromate l7 gll temperature 80 90 C bathing time 40 seconds.

We claim:

1. A method for currentless blackening copper-containing aluminum alloys comprising immercing said aluminum alloy for a period of less than about 1.5 minutes in an aqueous bath containing quantities of alkali metal hydroxides and alkali metal bichromates corresponding to from 8-50 g/l of sodium hydroxide and from 10-20 g/l of potassium bichromate, the bath temperature maintained at about 80 to about 100 C.

2. The method as claimed in claim 1 wherein from 30-40 g/l of an alkali metal carbonate is included in said aqueous bath.

3. The method as claimed in claim 2 wherein said immersio'n is conducted for a period from 1 second to l minute in an aqueous bath maintained at a temperature at about to about C, said aqueous bath consisting essentially of about 35 g/l of sodium carbonate, about 15 g/l of potassium bichromate and about 10 g/l of sodium hydroxide.

4. A method of applying an even, tenacious black coating on copper-containing aluminum alloys in the absence of current, said coating having an emission coefficient of at least 0.8 for a temperature radiation between 20 C. and 100 C., comprising immersing said aluminum alloy in an aqueous bath consisting essentially of:

alkali metal hydroxide 8-50 g/l alkali metal bichromate 10-20 g/l alkali metal carbonate 30-40 g/l at a temperature of about 80 to about 100 C for a period of less than about 1.5 minutes.

5. An aqueous bath for currentless blackening of copper-containing aluminum alloys by applying thereto a black coating, said bath containing 8-50 g/l of an alkali metal hydroxide, 10-20 g/l of an alkali metal bichromate and 30-40 g/l of an alkali metal carbonate.

6. A blackened aluminum alloy article having an even, tenacious black coating thereon, said coating having an emission coefficient of at least 0.8 for a temperature radiation between 20 C. and 100 C., applied according to the method'of claim 1. 

2. The method as claimed in claim 1 wherein from 30-40 g/l of an alkali metal carbonate is included in said aqueous bath.
 3. The method as claimed in claim 2 wherein said immersion is conducted for a period from 1 second to 1 minute in an aqueous bath maintained at a temperature at about 95 to about 100* C, said aqueous bath consisting essentially of about 35 g/l of sodium carbonate, about 15 g/l of potassium bichromate and about 10 g/l of sodium hydroxide.
 4. A method of applying an eVen, tenacious black coating on copper-containing aluminum alloys in the absence of current, said coating having an emission coefficient of at least 0.8 for a temperature radiation between 20* C. and 100* C., comprising immersing said aluminum alloy in an aqueous bath consisting essentially of: alkali metal hydroxide 8-50 g/l alkali metal bichromate 10-20 g/l alkali metal carbonate 30- 40 g/l at a temperature of about 80* to about 100* C for a period of less than about 1.5 minutes.
 5. An aqueous bath for currentless blackening of copper-containing aluminum alloys by applying thereto a black coating, said bath containing 8-50 g/l of an alkali metal hydroxide, 10-20 g/l of an alkali metal bichromate and 30-40 g/l of an alkali metal carbonate.
 6. A blackened aluminum alloy article having an even, tenacious black coating thereon, said coating having an emission coefficient of at least 0.8 for a temperature radiation between 20* C. and 100* C., applied according to the method of claim
 1. 